1. Field of the Invention
The invention relates to a drying and/or a fixing device for use with a textile material web. In particular, the invention relates to blowboxes wherein a ventilator (with a pressure box) is arranged on one longitudinal side of the device, and the inlet of a suction box is arranged on the opposite longitudinal side of the device.
2. The Prior Art
Many types of treatment devices are known for treating material webs. These types of devices include numerous designs such as plane dryers and tenters. In practice, almost all of these treatment devices have ventilating systems which are designed so that the escaping treatment medium is carried away from the web. This, however produces unstable current conditions which lead to disturbing fluttering or buffeting of the web under treatment. The waste gas current components of the treatment medium cause unfavorable dynamic tensioning or pressure loads on the web. There are even occasional contacts between the web and the discharge nozzles.
In one design, both the ventilators and associated blowboxes may be positioned below the treatment plane on the same longitudinal edge of the device (U.S. Pat. No. 5,203,094) or on opposite longitudinal edges (DE 27 54 438 Al). In addition, the ventilators associated with the two blowboxes may be integrated into each other (DE 22 01 731 Al). Furthermore, the two blowboxes may also be connected to the same ventilator (DE 36 27 904 C2). It is also known that a ventilator can be assembled with a blowbox both above and below the treatment plane (DE 29 08 348 Al). Finally, machines of the type specified above exist where the blowboxes are positioned near the floor of the device, and the ventilator is spatially arranged above the treatment plane. The treatment plane is arranged in between the blowboxes and the ventilator. In all cases, the treatment gases (or treatment medium) may be of any type. For the sake of simplification, the term "air" is often briefly used without limiting the generality.
In all of the above devices, the circulating air conducted in the ventilator-blowbox-ventilator circuit is blown toward the material web (viewed specifically across the width of the material web) with the same pressure everywhere. The air flowing away from the material web (following reflection or flow-through) is withdrawn into a reflux (or return flow) chamber connected to the suction side of the respective ventilator, by way of reflux or return flow ducts (e.g. blowbox holes or gaps in blowboxes designed in the form of fingers) provided in the blowboxes. The air flowing away from the material web is subjected to the vacuum generated by the suction side of the respective ventilator. The vacuum has different values across the width of the material web and such values correspond with the geometry of the external environment of the blowboxes (in the interior of the machine) and their spacing from the suction side of the ventilator.
The highest vacuum is found in the zone of the return flow opening of the suction box. This return flow opening is usually closed with a filter screen. For reasons of maintenance technology, the screen is generally disposed near a longitudinal edge on the side of the machine. Therefore, it is arranged unsymmetrically relative to the surface of the treated material web. Due to the unsymmetrical characteristics of the returning treatment gas, the forces of air acting on the material web in the zone of its longitudinal edges are different from those acting on the center of the web. Depending on the ratio between the air blown at the web from the top and the air blown at the web from the bottom (top air:bottom air), the material of the web "flutters" on the edge or sags there more than it does in the center. With sensitive or delicate articles, the mechanical stresses acting on the material in the edge zone (fluttering, sagging) result in partial lengthenings (the so-called bagging effect; formation of bags), which may be so strong that they cannot be tolerated.
If the material web is guided horizontally, the effect of the weight of the web (which changes, for example after drying) can be compensated by changing the pressure acting on the underside of the web relative to the other pressure, EP 0 471 162 Cl. However, the formation of bags or bagging cannot be eliminated by such adaptation of the pressure, which is the same everywhere across the width of the material. For compensating the forces causing the formation of bags it would be necessary to make the distribution of the pressure uneven along the width of the web according to the shape of the bag. This requires complicated measuring and controlling systems.
A drying machine comprising two ventilators mounted on a common axle is known in the art (DE-PS 78 282). This machine does not belong to the aforementioned group of tenter-type machines; however, with this machine, the inflow and the off-flow chambers for the treatment gas on both sides of the material web are both positioned symmetrically with respect to the center line of the treated material web. In practice, however, it is still not possible to achieve equal treatment of the material web at all points across the width of the web because the treatment gas which impacts the material web has both a vertical and horizontal component.